1. FIELD OF THE INVENTION
The present invention relates to a porous synthetic resin film having many fine communicating holes which is useful as a filter.
2. DESCRIPTION OF THE PRIOR ART
Heretofore, porous films used for an air permeable diaphragms, filters and the like include paper, glass fiber sheets, synthetic fiber sheets, cellulose derivative filters and the like. It is desired to use membranes having narrow hole size distribution for filters. However, commercial filters have broad hole size distribution. For example, filters having an average hole diameter of 5 .mu.m will pass particles having a diameter of 10 .mu.m. Typical of filter problems is where the communicating holes in the filter have various portions, each having different diameters. Accordingly, a particle may pass into one communicating hole, but may be caught at a part of the hole which has a smaller diameter than the hole opening which causes clogging to occur. The clogging cannot be prevented by stirring the solution on the surface of film and the life of film is remarkably decreased.
Accordingly, there exists a need for porous films having uniform, fine cylindrical communicating holes.